This invention relates to sintered silicon carbide bodies. More particularly, the invention relates to a process for forming silicon carbide articles.
Silicon carbide articles or bodies of simple geometry have been formed by the hot-pressing method. One such method is described by R. A. Alliegro et al., Journal of the American Ceramics Society, 39, page 386 (1965) and by Weaver et al in U.S. Pat. No. 3,836,673. According to Weaver highly dense silicon carbide bodies are formed by intimately mixing fine alpha silicon carbide powder and 0.5 to 5% by weight of fine aluminum powder, by ball milling the two materials in isopropanol, drying the mixture and passing it through a 40 mesh screen (U.S. Standard Sieve Series), loading the thusly prepared powder mixture into a graphite mold set-up, and hot-pressing the powder in an induction heated furnace using argon gas as a purge at a temperature of 2075.degree. C. at approximately 2700 p.s.i. The resulting product has a density essentially that of theoretical. Although silicon carbide does not readily lend itself to the straight forward conventional cold pressing and sintering process, because of the reluctance of the silicon carbide particles to sinter and densify, silicon carbide articles have been successfully made by modifications of this basic process. It has been found that the incorporation of minor amounts of certain metals e.g. boron or aluminum, facilitates the formation of relatively dense silicon carbide bodies by cold pressing a green shape followed by sintering.
Another fabrication technique is the so-called reaction sintering or reaction bonding method. According to this method a green body or perform of silicon carbide powder or silicon carbide and a carbonaceous material or carbon per se, is made by cold-pressing, extruding, isostatic pressing, casting, or the like. The green body is then heated in contact with molten silicon metal or silicon vapors which causes the silicon to enter the interstices of the preform. When carbon is present in the preform the silicon reacts with the carbon to form additional silicon carbide thus densifying the structure. C. W. Forrest et al., in Special Ceramics, 5, page 99 (1972) describes extensive work done with the reaction sintering technique. One of the variables studied by these workers was the effect of the particle size of the starting silicon carbide powder. Their data showed that particle size had a very pronounced effect on mechanical strength down to a particle size of about 100 microns. However below that particle size the effect on the mechanical properties was much less pronounced.
Several variants of the basic reaction sintering process have been described in the patent literature. According to J. C. Andersen in U.S. Pat. No. 2,938,807 dense silicon carbide bodies are formed, containing less than 5% free silicon, by cold pressing a uniform mixture of silicon carbide powder, a carbonaceous material e.g. graphite, and a carbonizable material which functions as a temporary binder as well as a donor to the total amount of carbon. The green shape is heated to carbonize the carbonizable material followed by a subsequent heat treatment at 2250.degree. C. in the presence of silicon metal, which results in the situ formation of silicon carbide in the pores of the original cold pressed shape. The particle size of the silicon carbide employed by Andersen was relatively coarse. The fine grit mixture which Andersen employed was made up, in terms of parts by weight, of 49.5 parts of 100 mesh (170 microns) silicon carbide, 13.1 parts of 220 mesh (70 microns), 12.3 parts of 20 micron material and 8.2 parts of 6-7 micron material; the numerical average particle size of this fine mixture is relatively coarse. A modified reaction sintering method is disclosed by K. M. Taylor in U.S. Pat. No. 3,205,043 wherein a green silicon carbide preform is cold pressed from silicon carbide powder containing a small quantity of a temporary organic binder, for example a phenolic resin. The preformed body is fired at 2300.degree. C. by way of a gradual temperature rise from room temperature, which causes the decomposition and removal of the temporary binder and the recrystallization of the silicon carbide. The resulting porous structure is then impregnated with a carbonizable material such as a furfural based compound or a phenol-aldehyde resin, and subsequently heat treated to carbonize the organic material. The impregnation with the organic material and the subsequent carbonizing heat treatment are repeated until the carbon content of the sintered silicon carbide preform is 85 to 95% of what is needed to react with the theoretical amount of silicon needed to fill essentially all of the pores. The carbon laden silicon carbide structure is then contacted with silicon metal at about 2200.degree. C. The silicon melts and penetrates the structure, reacting with the carbon to form silicon carbide. Like Andersen, Taylor's silicon carbide powder is multi-modal and relatively very coarse. The finest silicon carbide mixture exemplified by Taylor has the following make-up on a parts by weight basis: 55 parts 100 mesh (170 microns), 15 parts of 220 mesh (70 microns) 15 parts of 3F mesh, and 15 parts of 1000 mesh.
According to P. Popper in U.S. Pat. No. 3,275,722 dense self-bonded bodies of silicon carbide result from preforming a mixture of silicon carbide powder, carbon for example in the form of colloidal graphite, and a temporary binder. Although Popper teaches exposing the preform to silicon within the temperature range of from 1800.degree. to 2300.degree. C., his preferred teaching is the heat treatment in vacuum of the preformed material in contact with silicon at only 1500.degree. C. The particle size of the silicon carbide employed by Popper is relatively very coarse with a typical mix, including graphite, having the following weight percent makeup: 43% 80 mesh (240 microns), 11% 220 mesh (70 microns) and 18% 700 mesh (less than 16 microns) silicon carbide admixed with 28% graphite.
Still a further variant is C. W. Forrest process described in U.S. Pat. No. 3,495,939. Forrest forms a porous, coherent body of powdered silicon carbide and carbon which can originate from colloidal graphite, heats the preformed shape at from 1600.degree. to 1700.degree. C. in an environment of silicon monoxide vapor which increases the surface porosity of the body, and while heating, contacts at least one surface of the body with molten silicon metal. Capillarity draws the molten silicon into the pores of the body forming silicon carbide therein.
A further advancement in the art is set out in G. Q. Weaver's pending application Ser. No. 555,855, filed Mar. 6, 1975. The application discloses a process particularly directed at the fabrication of complex silicon carbide shapes. A green set of billets is formed by any of the several well known techniques including hot-pressing. If the billet is cold formed it is then lightly sintered at a temperature of 1500.degree. to 2000.degree. C. Alternatively the light sintering may be accomplished by directly hot-pressing the silicon carbide powder. In addition to sintering this treatment causes partial densification. A billet or slab is then shaped to the desired configuration by machining, scraping, grinding or the like. The shaped body is then fully sintered and densified by a second heat treatment at a temperature of about 2000.degree. C. which further and finally densifies the structure. Alternatively the foregoing procedure may include and preferably does include, the inclusion in the original silicon carbide powders of 5 to 40% by weight of carbon or the carbon may be added after the initial sintering step in the form of a carbonizable material as disclosed in the Taylor Patent U.S. Pat. No. 3,205,043. In this latter case the carbon-containing presintered silicon carbide body is then exposed to silicon metal at a temperature of 1500.degree. to 2100.degree. C. in a nitrogen atmosphere, which causes the silicon to permeate the silicon carbide structure and react with the carbon therein to form silicon carbide in situ and resulting in full and final densification of the shaped article. The particles size of the original silicon carbide powder is preferably approximately 3 microns. The silicon carbide however may be bimodal in makeup in which case at least 50% of the silicon carbide powder has an average particle size of 3 microns or less while the remainder of the silicon carbide may be from 30 to 170 microns.